JP2002363767A - Welded steel pipe having excellent corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a welded steel pipe which has excellent corrosion resistance, has satisfactory workability, resistance weldability and adhesion for a coating film as well. SOLUTION: A welded steel pipe made from a galvanized or galvannealed steel sheet is used as a base material, and, via a dense boundary reaction layer 3 having an excellent environmental shielding function, a chemical conversion treatment film 4 in which oxide or hydroxide grains of valve metal are dispersed into a resin matrix 4a is formed on a plated layer 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a welded steel pipe which is suitable for agricultural facilities and buried pipes which are exposed to wind and rain for a long period of time and which has little influence on the environment and has excellent corrosion resistance.

[0002]

2. Description of the Related Art Welded steel pipes made of plated steel plates are used for materials that are exposed to the wind and rain for a long period of time, such as frameworks of vinyl houses, and pipes buried in soil. Rust prevention treatment is required to improve the corrosion resistance and durability of welded steel pipes, but even if an inorganic or organic treatment is applied to the plated steel sheet before pipe making, the coating will be damaged by galling etc. during pipe making, Alternatively, the sprayed layer that has repaired the weld remains exposed. As a result, corrosion originating from the damaged part of the coating or the repaired part by thermal spraying occurs.
It grows, and the appearance as well as the initial white rust or the red rust after long-term use deteriorates the function of the structure as well as the appearance.

[0003] For this reason, a rust preventive treatment in which a rust preventive oil or a petroleum resin is applied to the surface of a welded steel pipe formed is known.
However, in this method, the corrosion resistance is not sufficiently improved, and the application of a rust-preventive oil or a petroleum-based resin tends to avoid stickiness. In this regard, the method of coating the aqueous resin has no stickiness and maintains a good appearance after the rust prevention treatment. However, if the processing oil remains on the surface of the steel pipe, the applied aqueous resin is repelled and a uniform resin film is not formed, so that the base steel is easily exposed. In addition, the water-based resin often lacks reactivity with the base steel, and thus has poor adhesion and high corrosion resistance.

Therefore, a method of improving the corrosion resistance by applying an organic resin containing a chromic acid compound as a rust inhibitor to the surface of a steel pipe has been adopted. The chromate film is a film composed of an oxide or hydroxide in which trivalent Cr and hexavalent Cr are combined. The hardly soluble trivalent Cr compound exhibits an environmental blocking function, prevents corrosion of the base material, and improves the adhesion of the resin film to the underlying plating metal. Hexavalent Cr is
It is eluted from the chemical conversion coating film as an oxyacid anion, and undergoes a reduction reaction with an exposed portion of the steel sheet formed by processing or the like to re-deposit as a hardly soluble trivalent Cr compound. By the precipitation of trivalent Cr, the chemical conversion treatment film is self-repaired, and an excellent anticorrosion action is exhibited.

[0005]

However, the chromate treatment imposes a great burden on the drainage treatment containing Cr ions. In addition, when used for a long time in an underground pipe or the like, hexavalent Cr harmful to the environment may be eluted from the coating. Therefore, recently, various Cr-free treatments of titanium, zirconium, and phosphate have been studied. The present inventors also formed a chemical conversion treatment excellent in corrosion resistance when forming an interface layer and a film in which oxides or hydroxides such as Ti, Zr, He, V, Nb, Ta, Mo, and W and fluoride coexist. It has been proposed in Japanese Patent Application No. 2000-38867 that a steel sheet can be obtained.

[0006]

SUMMARY OF THE INVENTION The present invention has been found in the course of investigating the usefulness of a chemical conversion treatment film proposed in the prior application, and a chromium-free treatment solution instead of a chromate treatment solution has been developed. It is an object of the present invention to provide a welded steel pipe having improved corrosion resistance by rust prevention treatment after pipe making use of its high reactivity with a thermal spray repair layer.

In order to achieve the object, a welded steel pipe of the present invention is based on a welded steel pipe formed from a galvanized steel sheet or a zinc alloy-plated steel sheet, and has an oxide having a high insulation resistance. Alternatively, a chemical conversion coating in which an oxide or hydroxide of a valve metal and a fluoride are dispersed in an organic resin is formed through an interface reaction layer in which a hydroxide and a fluoride coexist.

[0008] As the valve metal, Ti, Zr, H
f, V, Nb, Ta, Mo, W and the like. The concentration ratio F / O of O and F contained in the chemical conversion coating is 1/1 by atomic ratio.
When fluoride is contained so as to be not less than 00, the self-healing action caused by the fluoride becomes remarkable. Examples of the organic resin in which the oxide or hydroxide of the valve metal and the fluoride are dispersed include olefins such as urethane, epoxy, polyethylene, polypropylene and ethylene-acrylic acid copolymers, styrenes such as polystyrene, and polyesters. ,
Alternatively, one or two or more resins selected from copolymers or modified products thereof and acrylic resins are used.

The chemical conversion coating may further contain a soluble or hardly soluble metal phosphate or composite phosphate. Examples of the soluble metal phosphate or composite phosphate include salts of alkali metals, alkaline earth metals, Mn, and the like. Examples of poorly soluble metal phosphates or complex phosphates include Al, T
There are salts such as i, Zr, Hf, and Zn. This chemical conversion coating film was prepared by converting a chemical conversion treatment solution prepared to a predetermined composition to a coating amount of 0.1%.
It is produced by applying to the surface of a welded steel pipe at a coating amount of up to 3.0 g / m ² and then drying at a sheet temperature of 50 to 250 ° C. without washing with water. Prior to the application of the chemical conversion treatment solution, the plating layer such as a weld portion damaged during welding is formed of Zn, Zn-A
The repair treatment is carried out by spraying an alloy, Al or the like, but the applied chemical conversion treatment liquid sufficiently reacts also on the spray repaired portion, and forms a uniform anticorrosion film on the surface of the welded steel pipe.

[0010]

In the welded steel pipe according to the present invention, as shown in the schematic diagram of FIG. 1, a chemical conversion coating 4 is formed on the surface of a plating layer 2 on a steel substrate 1 via an interface reaction layer 3. . The interface reaction layer 3 and the chemical conversion coating 4 are similarly formed on the thermal spray repaired portion where the oxide film has grown thick. Interface reaction layer 3
Are zinc fluoride, zinc phosphate formed by the reaction of the plating layer 2 with the fluoride, phosphate, etc. contained in the chemical conversion treatment solution;
It is a dense layer made of reaction products such as fluorides and phosphates of valve metal and exhibiting excellent environmental shielding ability. In the chemical conversion coating 4, an oxide or hydroxide 4b of a valve metal and a fluoride 4c are dispersed in a resin matrix 4a in a granular form. Depending on the composition of the chemical conversion treatment solution, phosphate, composite phosphate, and the like may be further dispersed in the chemical conversion coating 4.

Since the chemical conversion coating 4 is formed via the dense interface reaction layer 3, the corrosive components in the atmosphere are prevented from directly reaching the steel substrate 1. In addition, since particles such as oxides or hydroxides 4b, fluorides 4c, phosphates, and composite phosphates of the valve metal are three-dimensionally dispersed in the chemical conversion coating 4 which is thickened with an organic resin. In addition, corrosive components such as moisture penetrating the resin matrix 4a are captured by the dispersed particles, and the corrosive components reaching the interface layer 3 are significantly reduced. Therefore, an excellent anticorrosion effect can be obtained in combination with the dense interface reaction layer 3.

Further, since the chemical conversion coating 4 contains a flexible organic resin, the chemical conversion coating 4 follows plastic deformation of the material at the time of forming a welded steel pipe, and cracks are greatly generated. To decrease. Furthermore, since the organic resin of the chemical conversion coating 4 has a high affinity for the resin coating amount, the coating adhesion is also improved, and when the film thickness is 3 μm or less, the pick-up of the plating component to the Cu-based electrode is suppressed. It is also effective in improving weldability.

The valve metal refers to a metal in which an oxide exhibits high insulation resistance, and includes Ti, Zr, Hf, V, Nb, Ta,
One or more of Mo and W are used. The coating made of the oxide or hydroxide of the valve metal acts as a resistor against the transfer of electrons, and suppresses the reduction reaction (oxidation reaction with the underlying steel) due to dissolved oxygen contained in the moisture in the atmosphere. As a result, elution (corrosion) of the metal component from the base steel is prevented. Above all, I such as Ti, Zr, Hf
The tetravalent compound of group V element A is a stable compound and is a suitable film component because it forms an excellent film.

When the oxide or hydroxide of the valve metal is formed on the surface of the welded steel pipe as a continuous film, it effectively acts as a resistor against electron transfer. The occurrence of film defects during processing is inevitable. Since the underlying steel is exposed at the film defect, the expected corrosion inhibiting action cannot be expected. Therefore, in the present invention, a self-repairing action is imparted to the chemical conversion treatment film by allowing the soluble fluoride of the valve metal to coexist. The fluoride of the valve metal dissolves in the moisture in the atmosphere and then re-precipitates as a hardly soluble oxide or hydroxide on the surface of the base steel exposed from the defective film portion, thereby filling the defective film portion. Exhibits self-healing action.

For example, a titanium-based film formed on the surface of a welded steel pipe has an oxide [TiO ₂ ] or a hydroxide [Ti (O
H) ₄ ] is a composite film. Microscopically, the base steel is exposed at portions where the film thickness is extremely insufficient or at film defects such as pinholes, and is likely to be a starting point of corrosion. In this regard, in the conventional chromium-based coating, self-healing action is exhibited by soluble hexavalent Cr precipitated as a poorly soluble trivalent Cr compound in the coating defect portion, but the self-healing action cannot be expected in the titanium-based coating. By increasing the film thickness, the film defects can be reduced, but the titanium-based film, which is hard and has poor ductility, cannot follow the elongation of the welded steel tube when forming a chemical-treated welded steel tube, and causes cracks, galling, etc. Defects tend to occur in the chemical conversion coating.

On the other hand, X _n TiF ₆ (X: alkali metal, alkaline earth metal or NH ₄ , n = 1 or 2), T
When a fluoride such as iF _{4 is} allowed to coexist, the fluoride is eluted from the chemical conversion treatment film and TiF ₆ ²⁻ + 4H ₂ O → Ti (OH) ₄ +
6F ^- reaction by re-precipitation in the coating defect becomes an oxide or hydroxide of poorly soluble, such as, exhibit self-healing effect.
The fluoride may be the same or different from the metal to be an oxide or hydroxide. In addition, when Mo or W is selected as a valve metal, some of these hexavalent oxyacid salts have a solubility, and some exhibit a self-healing action. Therefore, restrictions on the fluoride contained in the chemical conversion coating are reduced.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As an original plate for a welded steel pipe, a zinc or zinc alloy plated steel plate manufactured by an electroplating method, a hot-dip plating method, or a vapor deposition plating method is used. For zinc alloy plating, Zn-Al, Zn-Mg, Zn-Ni, Zn-Al
-Mg and the like. Further, an alloyed galvanized steel sheet subjected to alloying treatment after hot-dip plating can also be used as a base plate for chemical conversion treatment. The plated steel sheet is formed into a welded steel pipe of a predetermined size according to a conventional method such as high frequency welding or laser welding. The obtained welded steel pipe is subjected to a chemical conversion treatment, and the chemical conversion treatment may be any of a coating type and a reaction type. However, in the reaction-type chemical conversion treatment, the pH is adjusted slightly lower in order to maintain the stability of the treatment liquid. In the following description, Ti is taken as an example of a valve metal, but the same applies when a valve metal other than Ti is used.

The chemical conversion treatment solution contains a soluble halide or oxyacid salt as a Ti source. Ti fluoride is Ti
Although it is effective as a source and an F source, (NH ₄ ) F
In some cases, such a soluble fluoride as F source is separately added to the chemical conversion treatment solution. As a specific Ti source, K
_n TiF ₆ (K: alkali metal or alkaline earth metal,
n: 1 or 2), K ₂ [TiO (COO) ₂ ], (NH ₄ ) ₂ Ti
_{F 6, TiCl 4, TiOSO 4} , Ti (SO 4) 2, Ti (O
H) _Fourth magnitude. The mixing ratio of each component of the Ti source is selected so that a chemical conversion coating composed of an oxide or hydroxide and a fluoride having a predetermined composition is formed when the titanium source is dried and baked after applying the chemical conversion treatment liquid. You.

In order to stably maintain the Ti source as ions in the chemical conversion treatment liquid, it is preferable to add an organic acid having a chelating action. When an organic acid is added, the molar ratio of organic acid / metal ion is determined to be 0.02 or more, since the chemical conversion treatment solution is stabilized by chelating metal ions. Examples of the organic acid include tartaric acid, tannic acid, citric acid, oxalic acid, malonic acid, lactic acid, and acetic acid. Among them, oxycarboxylic acids such as tartaric acid, and polyhydric phenols such as tannic acid stabilize the processing solution and exhibit the function of complementing the self-healing action of fluoride, which is also effective in improving the coating film adhesion. It is. Orthophosphates or polyphosphates of various metals may be added in order to include soluble or hardly soluble metal phosphates or composite phosphates in the chemical conversion coating.

The soluble metal phosphate or composite phosphate elutes from the chemical conversion treatment film and elutes at the film defect, and reacts with the Zn, Al, etc. of the plating layer to precipitate insoluble phosphate. Complements the self-healing action of titanium fluoride. Further, since the atmosphere is slightly acidified when the soluble phosphate dissociates, the hydrolysis of titanium fluoride and, consequently, the formation of poorly soluble titanium oxide or hydroxide are promoted. Metals that form soluble phosphates or complex phosphates include alkali metals, alkaline earth metals, Mn, etc., and various metal phosphates or various metal salts and phosphoric acid, polyphosphoric acid,
It is added to the chemical conversion treatment solution as a phosphate. The hardly soluble metal phosphate or composite phosphate is dispersed in the chemical conversion coating,
Eliminates film defects and improves film strength. Al, Al, may be used as the metal forming the hardly soluble phosphate or composite phosphate.
There are Ti, Zr, Hf, Zn, etc., which are added to the chemical conversion treatment solution as various metal phosphates or various metal salts as phosphoric acid, polyphosphoric acid, and phosphate.

In a welded steel pipe formed from a plated steel sheet on which a plating layer containing Al is formed, black discoloration easily occurs. In this case, one or more metals selected from Fe, Co, and Ni are used. Black discoloration can be prevented by the presence of a salt in the film. In the case where a large crack has occurred in the plating layer due to severe processing or the like, the self-repairing action of fluoride or phosphate alone may not be sufficient. In this case, by providing a large amount of soluble hexavalent oxyacid salts of Mo and W in the coating, the same action as hexavalent chromium is developed to repair cracks in the plating layer and improve corrosion resistance. Further, in order to form the chemical conversion coating film 4 in which particles such as oxides or hydroxides 4b and fluorides 4c of the valve metal are dispersed in the resin matrix 4a, urethane-based, epoxy-based,
Olefins such as polyethylene, polypropylene, ethylene-acrylic acid copolymers, styrenes such as polystyrene, polyesters, or copolymers or modified products of these or organic resins such as acrylics are added to the chemical conversion treatment solution. .

As the urethane resin, a water-soluble or water-dispersible urethane resin obtained by reacting an organic polyisocyanate compound and a polyol compound, among which a self-emulsifying urethane resin is preferable. Examples of the organic polyisocyanate compound include aliphatic diisocyanates such as phenylene diisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate, and naphthalene diisocyanate, and alicyclic diisocyanates such as cyclohexane diisocyanate, isophorone diisocyanate, norbornane diisocyanate, xylylene diisocyanate, and tetramethyl xylylene diisocyanate. No. On the other hand, examples of the polyol compound include polyolefin polyols such as polyester polyol, polyether polyol, polycarbonate polyol, polyacetal polyol, polyacrylate polyol, polyesteramide polyol, and polybutadiene.

A self-emulsifying urethane resin is produced by introducing a hydrophilic component such as a carboxylic acid-containing compound into a molecule. In a self-emulsifying urethane resin, the ionicity becomes an anion, a cation, or a nonion depending on the introduced hydrophilic component. For example, introduction of polyethylene glycol, isocyanate, or the like into the polymer skeleton results in nonionicity, introduction of a hydroxyl group results in anionicity, and introduction of a sulfonic acid (salt) group, carboxyl (salt) group, or the like results in cationicity. In a chemical conversion treatment solution containing a phosphate, or a system to which an oxycarboxylic acid or a silane coupling agent is added, the anionic or cationic urethane resin may cause gelation due to an agglomeration reaction of the resin particles, but may have a nonionic property. Such a phenomenon does not occur in urethane resin. In this regard, the use of a nonionic urethane resin is preferable, but an anionic or cationic urethane resin can be used without causing gelation by setting the use ratio of the nonionic urethane resin to at least half.

In the chemical conversion treatment liquid, a wax effective for improving lubricity is contained in the chemical conversion treatment film. Agents and the like can also be added. It is considered that the organic wax having a low melting point bleeds on the surface when the film is dried, and exhibits lubricity. The high melting point organic wax and the inorganic lubricant are present in a dispersed state in the film, but it is considered that lubricity is exhibited by being exposed on the surface of the film in an island-like distribution on the outermost layer of the treated film.

As described above, the Ti source compound consisting of a halide or an oxyacid salt may be used, if necessary, to supplement the self-repairing action of an organic acid having an ion stabilizing action or the self-healing action of titanium fluoride to improve the corrosion resistance. An organic resin may be further blended based on a processing solution to which a certain phosphate is added. When the prepared treatment liquid is applied to the welded steel pipe, a coating layer that has reacted with the inorganic anions of fluorine ions and phosphate ions and the underlying plating metal or Ti is preferentially and densely formed on the plating layer surface, and is formed thereon. It becomes a two-layer film in which an organic resin film in which fluorides, oxides, hydroxides and phosphates of Ti are dispersed. A similar reaction proceeds to the sprayed repair layer of Zn, Zn-Al alloy, Al, etc., and a uniform two-layer film is formed on the surface of the welded steel pipe.

The prepared chemical conversion treatment solution is roll-coated,
By applying to a welded steel pipe by a spin coating method, a spraying method or the like and drying without washing with water, a chemical conversion coating film having excellent corrosion resistance is formed on the surface layer of the plating layer and the thermal spray repair layer. The application amount of the chemical conversion treatment liquid is preferably adjusted so that the titanium adhesion amount is 1 mg / m ² or more in order to secure sufficient corrosion resistance. Fluorescent X
When elemental analysis is performed with a line, ESCA, or the like, the concentrations of O and F contained in the chemical conversion treatment film are measured. When the relationship between the concentration ratio F / O (atomic ratio) calculated from the measured values and the corrosion resistance was investigated, the occurrence of corrosion starting from the film defect was significant at the concentration ratio F / O (atomic ratio) of 1/100 or more. Decreased to.
This is presumed to be due to the titanium fluoride having a self-healing action being contained in the chemical conversion coating in a sufficient amount.

The interface reaction layer 3 and the chemical conversion coating 4
Preferably have thicknesses of about 3 to 300 nm and about 0.1 to 3 μm, respectively. The interface reaction layer 3 exhibits sufficient environmental barrier ability when the film thickness is 3 nm or more.
If the film grows to a thickness exceeding m, cracks are likely to occur due to the stress applied during the forming process, and on the contrary, the corrosion resistance is reduced. The reaching of the corrosive component to the interface reaction layer 3 is significantly suppressed by the chemical conversion coating 4 having a thickness of 0.1 μm or more.
However, in the case of the chemical conversion coating 4 having a thickness of more than 3 μm, not only the effect of improving the corrosion resistance is saturated, but also the weldability is deteriorated. The thickness of the interface reaction layer 3 and the chemical conversion coating 4 is A
It can be measured by elemental analysis in the depth direction by ES or GDS, TEM observation and the like.

Although the chemical conversion coating can be dried at room temperature, it is preferable to keep the temperature at 50 ° C. or higher to shorten the drying time in consideration of continuous operation. However, if the drying temperature exceeds 250 ° C., the organic components contained in the chemical conversion coating may be thermally decomposed, and the properties imparted by the organic components may be impaired. After the formation of the chemical conversion coating, an organic coating having further excellent corrosion resistance can be formed. Examples of this type of film include urethane-based resins, epoxy resins, olefin-based resins such as polyethylene, polypropylene, and ethylene-acrylic acid copolymer, styrene-based resins such as polystyrene, polyesters, and copolymers or modified products thereof. Resin film of acrylic resin etc.
When provided on the chemical conversion coating with a thickness of μm, high corrosion resistance surpassing that of the chromate coating can be obtained. Alternatively, by providing a resin film having excellent conductivity on the chemical conversion treatment film, lubricity is improved and weldability is also provided. This type of resin film can be formed, for example, by a method of applying an organic resin emulsion by electrostatic atomization (Japanese Patent Publication No. 7-115002).

[0029]

[Example] A plate thickness of 1.2 mm and a coating weight on one side of 60 g /
After forming an m ² hot-dip Zn-6% Al-3% Mg alloy plated steel plate into an open pipe shape, both ends in the width direction were subjected to high frequency welding to produce a welded steel tube having a diameter of 20 mm. After bead cutting the weld that has been damaged during welding,
It was repaired by double spraying of n and Zn-Al. As the chemical conversion treatment liquid, a Ti source and an F source were mixed with a urethane resin emulsion, and various metal compounds, organic acids, and phosphates were added depending on the case, and a chemical conversion treatment liquid having the composition shown in Table 1 was prepared.

[0030]

After the welded steel pipe was washed with warm water, the chemical conversion treatment solution shown in Table 1 was applied and treated with a sponge to adjust the amount of the chemical conversion treatment solution, and thus the film thickness of the chemical conversion coating film 4.
The welded steel pipe to which the chemical conversion treatment solution was applied was dried at 80 ° C. using a dryer without washing. The thickness of the interface reaction layer 3 and the chemical conversion coating 4 formed on the surface of the plating layer 2 were measured and the components were analyzed. Table 2 shows the measurement results.

[0032]

A test piece including a thermal spray repaired portion was cut out from each of the chemically treated welded steel pipes and subjected to the following various tests. Accelerated corrosion test Seal the cut end face of a 150 mm long test piece and apply JIS
Salt spray test according to Z2371 for 24 hours and 7
After continuing for 2 hours, the area ratio of white rust generated on the test piece surface was measured. From the measurement results, the corrosion resistance was evaluated as ◎ when the white rust generation area ratio was less than 5%, ○ when 5 to 10%, Δ when 10 to 30%, and × when 30% or more.

Corrosion test of flawed part After simulating a cross cut with a cutter on the surface of the test piece assuming flaws when handling the welded steel pipe, the same salt spray test 24
After a time, the width of the corrosion generated on the test piece surface was measured. From the measurement results, the maximum corrosion width was 2 mm or less ◎, 2 to 4 mm
Was evaluated as 〜, 4 to 8 mm as Δ, and 8 mm or more as ×, to evaluate the corrosion resistance of the flawed portion. As can be seen from the investigation results in Table 3, the welded steel pipes treated with the treatment liquids 1 to 7 according to the present invention exhibited excellent corrosion resistance in both the pipe body and the flawed portions.

On the other hand, the welded steel pipe treated with the treatment liquid 8 containing no fluoride was inferior in corrosion resistance at the flawed portion and lacked a self-healing action. Treatment liquid 9.1 without titanium compound
The welded steel pipe treated with No. 0 was inferior in long-term corrosion resistance as compared with the examples of the present invention. In a welded steel pipe in which a urethane resin film was simply formed by using the treatment liquid 11 or a welded steel pipe in which the rust-preventive oil of the treatment liquid 12 was simply applied, the corrosion resistance of both the steel pipe body and the flawed portion was poor. Although the chromate treatment liquid 12 imparts the same corrosion resistance to the welded steel pipe as in the present invention, Cr elution of 3 mg / m ² or more was detected in a Cr elution test.

[0036]

[0037]

As described above, the welded steel pipe according to the present invention provides a valve metal oxide or hydroxide via a dense interface reaction layer in which metal oxides or hydroxides and fluorides coexist. In addition, a chemical conversion coating in which fluoride particles are dispersed in a resin matrix is formed on the plating layer surface. Due to the high reactivity of the chemical conversion treatment liquid, a similar chemical conversion treatment film is formed even in the thermal spray repair portion where the oxide film is formed relatively thick. Therefore, the excellent environmental barrier function of the interface reaction layer is utilized, and the welded steel pipe has excellent corrosion resistance. In addition, since the chemical conversion coating contains a flexible organic resin, cracks generated during the forming process are small, and it is possible to process into a predetermined shape while suppressing the introduction of defects into the chemical conversion coating. Moreover, since it is a chemical conversion coating that does not contain Cr, which may adversely affect the environment, it is used in a wide range of fields as a material that replaces the conventional chromate-treated steel sheet.

[Brief description of the drawings]

FIG. 1 is a schematic view of a chemical conversion coating formed according to the present invention 1: steel substrate 2: plating layer 3: interface reaction layer
4: chemical conversion coating 4a: resin matrix 4b: oxide or hydroxide of valve metal 4c: fluoride

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yukihiro Morita 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. (72) Inventor Tadashi Tadashi Nakano 5 Ishizu Nishimachi, Sakai City, Osaka Nissin Steel Co., Ltd. In Technical Research Institute (72) Inventor Koichiro Ueda 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture F-Term in Nisshin Steel Co., Ltd.Technical Research Laboratory CA39 EB08 4K031 AA01 AB02 AB09 BA06 BA08 CB31 CB37 FA07 FA09 4K044 AA02 AB03 BA10 BA12 BA17 BA20 BA21 BB04 BC02 CA11 CA16 CA53

Claims (6)

[Claims] 1. An interfacial reaction in which an oxide, a hydroxide and a fluoride of a valve metal exhibiting high insulation resistance coexist with a welded steel pipe formed from a galvanized steel sheet or a zinc alloy coated steel sheet as a base material. A welded steel pipe having excellent corrosion resistance, characterized in that a chemical conversion coating in which an oxide or hydroxide of a valve metal and a fluoride are dispersed in an organic resin is formed through a layer. 2. The method according to claim 1, wherein the valve metal is Ti, Zr, Hf, V,
The welded steel pipe according to claim 1, wherein the pipe is at least one selected from Nb, Ta, Mo, and W. 3. The organic resin is one or more selected from urethane, epoxy, olefin, styrene, polyester, and acrylic resin, or a copolymer or modified product of these resins. Item 4. A welded steel pipe according to Item 1. 4. The welded steel pipe according to claim 1, wherein the concentration ratio F / O of O and F contained in the chemical conversion treatment film is 1/100 or more in atomic ratio. 5. The welded steel pipe according to claim 1, wherein the chemical conversion coating further contains a phosphate or a composite phosphate of a soluble or insoluble metal. 6. The method according to claim 1, wherein the plating layer of the welded portion is repaired by spraying Zn, Zn—Al alloy or Al alloy.
A welded steel pipe according to any one of claims 1 to 5.